Wind turbines are continuously growing in size, which increases the challenges                related to the construction of the wind turbines,        related to the mounting demounting, replacement, etc. of elements such as e.g. rotor blades, tower and tower elements, and        related to other activities for servicing the wind turbines, such as e.g. inspecting, repairing, cleaning, etc. the rotor blades.        
When building wind turbines, cranes, e.g. mobile cranes, cranes on trucks, lorries in case of land based constructions and ships, jack-up rigs, barges, etc. in case of sea-based constructions have usually been used and due to the increasing size of the wind turbines, such crane equipment needs to increase correspondingly in size, which increases costs correspondingly.
In connection with replacement of elements such as e.g. rotor blades, cranes have also been used for land-based as well as sea-based wind turbines, and it will be understood that this will increase costs considerably for such jobs, both because of the great size of the crane equipment and because the circumstances need to be suitable, when using such equipment, e.g. the weather condition in general and the wind condition in particular, especially when sea-based wind turbines are considered.
It has also been suggested to mount a service crane on a wind turbine, e.g. as disclosed in WO 2010/037392 A2, wherein it is disclosed that a service crane can be connected to the hub instead of a rotor blade. A similar example is disclosed by WO 2012/040534, which also discloses that a service crane can be mounted in place of a rotor blade or to a mount between adjacent rotor blades.
A further example of a service crane, which is mounted at a wind turbine is disclosed by EP 2 520 533 A1, according to which a service crane is mounted to the nacelle.
Even further, EP 1 101 934 A2 discloses a wind turbine having an on-board crane (or service crane) arranged in the nacelle of the wind turbine, which on-board crane comprises a frame having a pair of arms, which are pivotally arranged about a horizontal axis in the nacelle. Various loads, e.g. wind turbine parts, can be lifted up by means of a wire or line, which passes a roller arrangement at the outer end of the pair of arms and which is connected to a winch. The e.g. wind turbine parts can be lifted up via an opening in the bottom of the nacelle, when the pair of arms have been pivoted towards the rear end of the nacelle. When the pair of arms have been pivoted towards the front end of the nacelle, the pair of arms extends obliquely upwards and above the hub (or the hub area, in case the hub has not been installed), where the outer ends reach a position slightly in front of the nacelle and above the hub area. In this position, wind turbine parts such as rotor blades, the hub, etc. can be lifted up by means of the winch, the line or wire and the roller arrangement at the outer end of the pair of arms. In one embodiment, the frame having a pair of arms is pivotally arranged at the bottom of the nacelle with the winch arranged in the bottom of the tower of the wind turbine. In another embodiment the nacelle comprises a portal crane frame with a crane trolley, which crane trolley is movable along the portal crane frame above the components in the nacelle, e.g. above the generator, the gearbox, etc. In this another embodiment the pair of arms are arranged at the upper part of the portal crane frame at the front end, where the pair of arms are pivotally arranged about a horizontal axis. Also here, the pair of arms will extend obliquely upwards and above the hub (or the hub area, in case the hub has not been installed), when the pair of arms has been pivoted towards the front end of the nacelle, and the outer ends of the arms may reach a position slightly in front of the nacelle and above the hub area. Both of these embodiments furthermore require that the top of the nacelle can be opened, removed or the like, when it is intended to use the arrangement comprising the pivotable pair of arms.
It will be seen that in connection with both of the embodiments disclosed in EP 1 101 934 A2, that has been described above, the pivotally arranged pair of arms will in the forward position extend obliquely upwards, apparently in an attempt to clear the two rotor blades (in case the rotor blades are mounted) that will be located here. As it is well-known within the field of wind turbines, when the wind turbine is stopped, e.g. in case of service, repair, etc., the hub and the rotor blades are stopped in a position with one rotor blade pointing essentially vertical downwards and the other two (in case it is a three blade wind turbine, which is in most situations the case), pointing obliquely upwards in an approximately 60° direction in relation to the vertical direction. However, it will be seen that it may be difficult to arrange the pivotally arranged pair of arms in such a manner that they do not interfere with the rotor blades, thereby possibly damaging e.g. the surface of the rotor blades, if it is desired to have the outer ends reach forward of the nacelle. Further, it will be understood that such an arrangement with a pivotally arranged pair of arms may have to be designed specifically to the concrete type, size, etc. of wind turbine, in order not to collide with the rotor blades. Even further, the line or wire extending downwards from the outer ends of such a pivotally arranged pair of arms, which has been pivoted forwards, may be problematic e.g. since it may interfere with the hub and/or the rotor blade that is pointing downwards.
Further, it is noted that in connection with service operations performed on e.g. a rotor blade, service tools such as rotor blade guided tools, work platforms, etc. are used as disclosed for example in EP 1 583 905 B1, WO 03/048569 A2, WO 2005/064152 A2, WO 2007/085265 A1, WO 2009/155917 A2 and WO 2009/155918 A2, where such tools are connected for example by one or more lines or wires to fixing points at the top of the wind turbine, e.g. at the tower, at the nacelle or at the hub, and where the tools are being moved up and down a rotor blade pointing downwards by e.g. winches placed at the tools. However, as the wind turbines increase in size, the rotor blades increase correspondingly in size, which also means that, in particular due to the form of the rotor blades, a considerable distance in the horizontal direction between e.g. the tower and the rotor blade will be present, increasing towards the tip of the rotor blade. As the lines or wires are connected to the top of the wind turbine near the tower, it will be understood that the horizontal distance from the fixing point to the tool can be considerable, which may have undesirable effects, e.g. that the lines or wires are influenced by a force that deviates considerably from the vertical direction, that the tool will strain the rotor blade, that the tool, when applied, will have to contact or be entered onto the rotor blade at a relatively high level, e.g. where it is possible to reach the rotor blade, and move downwards along the whole length of the rotor blade, in case it is necessary to get near to the tip of the rotor blade, etc.